Acoustic emission (AE) sensors are used widely to capture and record stress waves in materials. These stress waves may be caused by changes in material loads or due to physical changes to the materials by heat or corrosion, or as the material approaches its yield point. AE sensors and signal processing techniques are used to detect leakage of a fluid or gas through a closed valve into a low pressure environment, such as another conduit, the atmosphere, or the ground in the case of a buried pipeline.
Various signal processing techniques and analysis methods have been employed to calculate or estimate a rate of the leakage. Units used for such measurements include liters/minute, gallons/minute, or mass leakage such as kilograms/second. There are at least four commercially available acoustic emission based products for calculating the leak rate based on the AE sensor signal. Three of those systems include the PAC 5131 and versions of the PAC 1278 ASL amplifier for on line monitoring and the SCORE MIDAS Meter. These systems employ similar broadband (e.g., 0 to 1 MHz) AE sensors and associated signal processing. Signal processing includes, for example, calculating and recording the average signal level (ASL) from the AE sensor. The ASL value can be used with lookup tables or in a simple algorithm to estimate the leak rate through the valve. The fourth commercially available product that uses acoustic emission to estimate valve leakage is the VALVEANALYZER marketed by Leak Detection Services (LDS). VALVEANALYZER uses similar broadband AE sensors but requires extended time domain recording of the output signal followed by conversion to the frequency domain using Fourier Transform processes. The analysis is performed manually by one skilled in the art of finding specific peak frequencies in the resulting frequency spectrum using electronic signature analysis techniques.
However, the conversion process from the AE signal level to actual leak rate through a valve with these prior art systems can be inaccurate. The PAC and SCORE products both provide a method of determining the error associated with the conversion process. The error calculated by PAC and SCORE at low confidence is +100/−50% of the calculated value. Thus, actual leakage can be anywhere between half and twice the calculated result. This level of inaccuracy is only available at low (i.e., 1 standard deviation) confidence. If higher confidence is required, such as 2 or 3 sigma, the actual leak rate could be up to 10 times the calculated value. The LDS system does not quote accuracy. It simply identifies the leak as red (large), yellow (not as large), and green (nonexistent or insignificant). High accuracy is required to be “Q-rated” for use in nuclear safety related applications. Accordingly, these existing leak detection systems would benefit from improved accuracy.